The present invention relates to traction drive fluids. More particularly, it is concerned with traction drive fluids which are of low viscosity, are of low volatility, have a high traction coefficient, have a good thermal stability, and are reduced in temperature dependency of viscosity and, therefore, which are suitable for use in continuously variable transmission of cars or industrial machines.
The term "traction drive fluid" as used herein means a fluid to be used in a traction drive (friction driving equipment utilizing rolling contact), such as a continuously variable transmission for cars or industrial machines, and a hydraulic machine. It is required for these traction drive fluids to have a high traction coefficient, high stability against heat and oxidation and, furthermore, to be inexpensive.
In recent years, the traction drive has been reduced in size and has often been used under high load conditions. In cases that the traction drive is used under such severe conditions, it is necessary to employ traction drive fluids which can exhibit satisfactory performance under various temperature conditions.
Various traction drive fluids have been proposed, including those as described in Japanese Patent Publication Nos. 338/1971, 339/1971, 35763/1972, 42067/1973, 42068/1973, 36105/1978, Japanese Patent Application Laid-Open Nos. 43108/1980, and 40726/1980. These traction drive fluids, however, have disadvantages in that viscosity is relatively high, causing a reduction in power transmission efficiency due to loss of stirring, and temperature dependency of viscosity is high. Various low viscosity compounds have been proposed in Japanese Patent Publication No. 339/1971. But these are not suitable for the lubricants due to these high volatility as these have low boiling points (e.g., dicyclohexylmethane: viscosity at 100.degree. F. 4.03 cSt, boiling point 250.degree. C.; 1,1-dicyclohexylethane: viscosity at 100.degree. F. 5.54 cSt, boiling point 260.degree. C., etc.)
It is necessary for the lubricants to have a boiling point over 300.degree. C. There haven't been yet, proposed traction drive fluids which have both low viscosity and low volatility. Thus it has been desired to develop traction drive fluids which are suitable for use in the traction drive.